Range estimation is used for a large number of application such as aerospace, defence, forensic science and automotive applications. The range may be estimated using transit time estimation of a transmitted signal, frequency comparison between a transmitted frequency modulated signal and the received echo, and multi frequency ranging.
Multi frequency ranging has the advantage that the range to an object can be determined with high accuracy without the need of complicated high precision timing equipment needed for transit time methods.
A Multi frequency ranging system transmits a signal comprising a primary frequency and a secondary frequency. The signal hits an object and the system receiver picks up the reflected signal. The relative phase, or the phase difference, between the primary frequency and the secondary frequency in the signal received by the system may be measured continuously. As the range increases the phase difference increases linearly modulo 360 degrees, with a slope proportional to the frequency difference between the primary frequency and the secondary frequency. This relation between the object range and the phase difference between two reflected signals is the basis for the multi frequency ranging technique.
In the case of a single secondary frequency the unambiguous range is limited to λ1=c/2(f1−f0), where F0 is the primary frequency, F1 is the secondary frequency, and c is the speed of the signal in the medium in which it propagates in e.g. the speed of light for an RF signal. This means that range values offset by an integer number of λ1 yields exactly the same phase difference.
The ambiguity is inversely proportional to the frequency difference, so by decreasing the distance between F1 and F0 the unambiguous range can be increased. However, range detection systems typically rely on filters protecting each receiver from being saturated by the adjacent carriers. Typically for radars, it is not feasible for F0 and F1 to be closer than approximately 800 kHz. Even if the filtering problem was solved, a small frequency difference yields a very noisy range measurement. Further, the difference between the frequencies has to be large enough to take an unknown Doppler frequency offset into account.
US20100103020 discloses a method of detecting a moving target within a predefined protected region with a microwave motion detector, by transmitting microwave frequency signals and receiving the microwave frequency signals reflected by a target. To determine the target distance without ambiguity, three or more microwave frequency signals may be transmitted at different frequencies.
In particular, the step of determining phase angles from the sampled intermediate frequency signal components comprises determining a first, second, and third phase from the sampled intermediate frequency signal components; the step of determining a phase difference between the phase comprises determining a first phase difference between the second phase and the first phase, and determining a second phase difference between the third phase and the first phase; and the step of determining, from the phase difference, a corresponding target distance measurement comprises: for each of the first and second phase differences, determining two corresponding distance measurements, wherein one distance measurement is a true distance measurement and the other distance measurement is an ambiguous distance measurement, and selecting an accurate distance measurement by matching the common true distance measurement of each phase difference.
Consequently, the unambiguous range can be increased.
However, in the presence of noise there exists the risk that an ambiguous range is selected as the true range. For an operator tracking the range to an object over time, this is perceived as a “range jump” e.g. the range to the object “jumps” from the true range to an “ambiguous” range or vice versa.
The risk of that happening depends on the chosen frequency set.
Thus it remains a problem to provide a method for evaluating the quality of frequency sets. Further, it remains a problem to provide a method for selecting a suitable frequency set, and a device configured to estimate the range to an object using a high quality frequency set.